Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
A cellular wireless network may include a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or a packet-switched network such as the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs.
A cellular wireless network may operate in accordance with a particular air interface protocol, with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, MMDS, WIFI, and BLUETOOTH. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and functions related to air interface communication.
In accordance with the air interface protocol, each coverage area may operate on one or more carrier frequencies and may define a number of air interface channels for carrying information between the base station and UEs. These channels may be defined in various ways, such as through frequency division multiplexing, time division multiplexing, and/or code-division multiplexing for instance.
By way of example, each coverage area may define a pilot channel, reference channel or other resource on which the base station may broadcast a pilot signal, reference signal, or the like that UEs may detect as an indication of coverage and may measure to evaluate coverage strength. As another example, each coverage area may define one or more uplink control channels or other resources on which UEs may transmit control messages to the base station. And each coverage area may define one or more downlink control channels or other resources on which the base station may transmit control messages to UEs. Each coverage area may then define one or more traffic channels or other resource for carrying communication traffic such as voice data and other data between the base station and UEs.
When a UE is operating in an active (e.g., connected) mode within coverage of a particular base station, the base station may receive a sequence of data from the network infrastructure for transmission over the air to the UE. As the base station receives that data, the base station may process the data through various layers of a protocol stack, ultimately modulating the data onto one or more carrier frequencies for transmission to the UE.
In an LTE network for instance, as the base station receives Internet Protocol (IP) packets for transmission to the UE, the base station processes the packets in a Packet Data Convergence Protocol (PDCP) layer to remove the relatively large IP packet headers and add token PDCP headers in their place, producing PDCP Protocol Data Units (PDUs). The base station then processes the PDCP PDUs in Radio Link Control (RLC) layer, segmenting the PDCP PDUs into a sequence of RLC PDUs with RLC headers. In turn, the base station then processes these RLC PDUs in a Medium Access Control (MAC) layer, which adds to each RLC PDU a MAC header and appropriate padding to fit the PDU into air interface transport blocks (e.g., 1 millisecond transmission time intervals) and manages output of the transport blocks to a physical layer for modulation and transmission in air interface subframes to the UE. Further, the MAC layer may engage in a hybrid automatic repeat request (HARD) process through which the MAC layer repeats transmission of data segments to the UE, incrementally adding error correction coding until the UE successfully receives the data.
As the UE receives this data transmitted from its serving base station, the UE may regularly monitor base station channel quality and provide the base station with channel quality reports, which the base station may then use as a basis to adjust data transmissions to the UE. For instance, the UE may regularly evaluate a signal-to-noise ratio and/or signal strength of the base station's pilot or reference signal and may periodically transmit to the base station a Channel Quality Index (CQI) based at least in part on that evaluation. As the base station receives these CQIs from the UE, the base station may then adjust characteristics of its data transmission to the UE based on the received CQIs, such as by selecting and applying a particular Modulation and Coding Scheme (MCS) corresponding with the latest received CQI.
At the same time, as the UE is being served in an active mode by a particular base station, the UE may also regularly monitor channel quality of one or more other base stations in the UE's vicinity, to determine whether handover to another base station may be appropriate. In particular, the UE may similarly evaluate signal-to-noise ratio and/or signal strength of the pilot or reference signal of each such other base station (or may do so at the direction of its serving base station), and the UE may determine whether that channel quality is threshold high (perhaps threshold higher than the channel quality from the UE's serving base station). If so, the UE may then signal to its serving base station to request handover to the other base station, and the serving base station may orchestrate the handover.
When an active UE hands over from one base station to another, i.e., from a source base station to a target base station, the network infrastructure may transition to sending data for the UE to the target base station for transmission over the air to the UE. Further, in some systems, to the extent the source base station has already received data for transmission to the UE, the source base station may transfer at least a portion of that data to the target base station so that the target base station can transmit that data over the air to the UE. In an LTE network, for instance, to the extent the source base station has data buffered in its RLC layer (pending transfer to the MAC layer), the source base station may transfer that data over an inter-base-station interface (“X2 interface) to the target base station for transmission to the UE. Once the UE receives that transferred data, then UE may then continue to receive additional data that the target base station receives from the network infrastructure for transmission to the UE.